1. Go to the Bitbucket repository you want to add users to.
2. On the left, go to Settings (the last icon on the bottom left).
3. Select Users and group access.
4. Under Users, start typing the name or email address of the user whom you want to add.
5. Select the privilege you want to grant that user (Read, Write or Admin).
6. Click Add.

Follow the same basic procedure to add a group instead of a user.

Now you have added more collaborators to your repository.